Create a Master File

To make a dd image master file of your customized Pi, insert your SD card into a USB adapter and plug it in to your Linux host. My Ubuntu laptop auto-mounts the partitions, although your system might not. Before making a dd image of the SD card, make sure the partitions on the card are not mounted:

$ sudo umount /dev/sda1
$ sudo umount /dev/sda2

Your SD card partition might not mount on /dev/sda, depending on how you computer is configured, so be sure you identify the correct device for your SD card.

Next, determine the last sector that the Raspberry Pi OS occupies on the SD card:

$ sudo fdisk -l

As shown in Figure 6, the last sector used by the second partition in this example is 4,173,823. Make note of this number, because you will need it when you enter the dd command.

Figure 6: Last sector of Pi OS.

Note that the dd command is unforgiving. It does exactly what you tell it, without annoying confirmation prompts. Use the command with care.

Now, create the image:

$ sudo dd if=/dev/sda of=/home/pi/pios_lite_2021-01-00_2GB.img count=4173822 status=progress

The if=/dev/sda (infile) parameter specifies that it reads from device /dev/sda, the of (outfile) parameter names the image file, the number after count is one less than the end sector noted in Figure 6, and the status parameter shows you what the command is doing.

Without using the count parameter, dd would copy the entire SD card, not just the desired Pi OS sectors. As shown in Figure 7, the command copied 2GB of data to the output file.

Figure 7: Creating an image with dd.

Success! You now have a custom Raspberry Pi OS image that burns quickly to an SD card of any size. It will automatically expand to the size of the target SD card and is ready to go on boot – no configuration required.

Remember to update the OS with

$ sudo apt update
$ sudo apt upgrade -y

when your custom image boots.

Summary

A custom Raspberry Pi operating system on an SD card can save you time and effort. I hope you found this guide useful and even learned a few things about the Raspberry Pi OS boot process along the way.